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LAB # 4 DC MACHINE CHARACTERIZATION In this lab you will be characterizing the DC machines included with your lab kit. You will assemble a DC dyno consisting of two DC machines connected at their shafts with a coupling. You will also calculate steady-state electrical and mechanical powers, and efficiency. Prelab For the prelab: 1. Mention the equation for the torque/speed constant Kr of the machine. 2. Derive an expression for the DC armature voltage when the angular velocity of the machine is zero (i.e., a "locked-rotor" condition). 3. Derive an expression for the DC armature current when the rotor is spinning at an angular velocity r, but there is no load torque (i.e., a “no-load” condition). Do NOT neglect friction torque produced by the rotational damping constant br. 4. Derive an expression for the efficiency of the dyno, where the input and output are the armature terminals of the DC machines. This expression should include the torque/speed constant Kr, estimate the armature winding resistance Ra, and the rotational damping constant br of the machines. Setup Assemble the DC machine dyno as shown in the video. Make sure the hex screws on the shaft coupling are sufficiently tightened to avoid slippage. Connect the power supply to one of the machines and apply 12V. The machine shafts should begin spinning. Adjust the machine mounts to align the shafts and minimize vibrations. Set the output voltage of the power supply to zero. Locked Rotor Test Attach the vise grips provided in the lab kit to one of the machine shafts so that the shafts cannot rotate. Slowly adjust the voltage and current knobs of the supply until its output current is 1A. Record the supply voltage. Continue to increase the supply output until its output current is 2A. Once again, record the supply voltage. You will use this data to estimate the armature winding resistance Ra of the machine. Set the output voltage and current of the power supply to zero. No-Load Test Remove the vise grips from the machine shaft. Adjust the output of the power supply until the output voltage is 4V. Record the output current of the supply. Using the multimeter provided with the lab kit, measure the terminal voltage of the machine that is NOT connected to the supply. Repeat for supply voltages of 8V and 12V. You will use this data to estimate the rotational damping constant br of the machines. Set the output voltage and current of the power supply to zero. Efficiency Test Using the alligator clips provided in the lab kit, connect the 50 resistor to the DC machine that is NOT connected to the power supply. Adjust the output of the supply until the output voltage is 4V. Record the output current of the supply. Also, use the multimeter to measure the output voltage of the DC machine connected to the load resistor. Calculate the input power (i.e., power from the supply), output power (i.e., the power absorbed by the resistor), and corresponding efficiency. Repeat for supply voltages of 8V and 12V. Postlab Report In your postlab report, you should present the estimated machine parameters, and compare your measured efficiencies with those predicted by the efficiency expression you derived in the prelab. Discuss any discrepancies.